mov: Only touch extradata in mov_read_extradata() if codec_id is what we expect

Extradata should only be parsed from the avss, fiel, jp2h and alac atoms for
AVS, MJPEG, Motion JPEG 2000 and ALAC respectively.
This also fixes the mov demuxer coming up with bogus extradata for some
AVC-Intra samples due to the presence of fiel atoms.